Components designed for the use in the area of high temperature, e.g. blades or vanes of a gas turbine, are usually coated with environmentally resistant coatings. The coating protects the base material against corrosion and oxidation due to the thermal effect of the hot environment and consists of an alloy mostly using the elements Al and Cr. Most turbine components are coated for the protection from oxidation and/or corrosion with, for example, a MCrAlY coating (base coat) and some are also coated with a Thermal Barrier Coating (TBC) for thermal insulation. MCrAlY protective overlay coatings are widely known in the prior art. They are a family of high temperature coatings, wherein M is selected from one or a combination of iron, nickel and cobalt. As an example, U.S. Pat. No. 3,528,861 or U.S. Pat. No. 4,585,481 disclose such kind of oxidation resistant coatings. U.S. Pat. No. 4,152,223, as well discloses such method of coating and the coating itself. Besides the γ/β-MCrAlY-coating, there is another class of overlay MCrAlY coatings which are based on a γ/γ′-gamma/gamma prime-structure, such as is disclosed in U.S. Pat. No. 4,973,445. The advantages of γ/γ′-coatings is that they have a negligible thermal expansion mismatch with alloy of the underlying turbine article. For higher thermal fatigue resistance the γ/γ′-coating are more convenient compared to the γ/β-type of MCrAlY-coatings. A higher thermal fatigue resistance in coatings is most desirable since failure of the most turbine blades and vanes at elevated temperature is typically thermal fatigue driven.
Among γ/γ′-coatings and γ/β-coatings, the field of γ/β-coatings have been an active area of research and a series of patents has been issued. E.g. a NiCrAlY coating is described in U.S. Pat. No. 3,754,903 and a CoCrAlY coating in U.S. Pat. No. 3,676,058. U.S. Pat. No. 4,346,137 discloses an improved high temperature fatigue resistance NiCoCrAlY coating. U.S. Pat. No. 4,419,416, U.S. Pat. No. 4,585,481, U.S. Reissue Pat. No. RE-32,121 and U.S. Pat. No. 4,743,514 describe MCrAlY coatings containing Si and Hf. U.S. Pat. No. 4,313,760 discloses a superalloy coating composition with good oxidation, corrosion and fatigue resistance.
Furthermore, in the state of the art Thermal Barrier Coatings (TBC) are known from different patents. U.S. Pat. No. 4,055,705, U.S. Pat. No. 4,248,940, U.S. Pat. No. 4,321,311 or U.S. Pat. No. 4,676,994 disclose a TBC-coating for the use in the turbine blades and vanes. The ceramics used are yttria stabilized zirconia and applied by plasma spray (U.S. Pat. Nos. 4,055,705 and 4,248,940) or by electron beam process (U.S. Pat. Nos. 4,321,311 and 4,676,994) on top of the MCrAlY bond coat.
Attempts have made in the literature in improving the adhesion of TBC by surface modification of the underlying bond coats. Briefly, U.S. Pat. No. 5,894,053 formed a rough surface on bond coat by applying a particulate metallic powders prior to ceramic thermal barrier coatings. The essential content of the patent is a process of forming a roughened surface by applying particulate materials on the bond coat using binder, and soldering powder. The disadvantages of the process could be the microstructural incompatibilities of the soldering materials with the coatings and thereby weakening the TBC interface at the Thermal Grown Oxide (TGO). In U.S. Pat. No. 4,095,003 a rough bond coat surface is formed by spraying a second layer of the bond coat using coarser plasma spray powders. In details the goal of U.S. Pat. No. 4,095,003 was to first provide a sealing layer to protect the substrate by a bond coat and then form a rough surface upon the bond coat by plasma spraying with coarse particles. Not considered was the formation of higher amount of transient oxides on the rough surface of MCrAlY coatings. These oxides are NiO and Cr2O3 including mixed oxides or spinel are formed during early oxidation. This observation is relevant to the TGO formed on the bond coat. The transient oxides formed are in contact with the TBC thusly weakening the interface.
Similar concepts of surface roughening were also used by U.S. Pat. No. 5,403,669; U.S. Pat. No. 5,579,534. In U.S. Pat. No. 5,403,669 the substrate is coated with a bond coat, then a rough bond coat is formed by plasma spraying then over aluminising the bond coat which is followed by TBC deposition. In U.S. Pat. No. 5,866,271 formed the rough surface on the superalloy substrate itself by either grit blasting, water jet blasting, plasma etching or atmospheric plasma spraying followed by aluminising or Pt aluminising of the surface prior to TBC application. In U.S. Pat. No. 6,242,050 formed the rough surface on the bond coat by application of powder using aluminum-silicon slurry. In yet in another patent U.S. Pat. No. 6,264,766 produced the rough surface by interwoven wires followed by metallic slurry coatings on the interwoven wires.
The rough surface tends to form transient oxides easily during early oxidation. The transient oxides are NiO and Cr2O3 and mixed oxides, i.e., spinel. Similarly, the rough surface formed by plasma spraying with coarse particles tends to form transient oxides during early oxidation. These transient oxides constituting the upper surface of the TGO is a weak point in the adhesion of TBC at the interface. The preferred oxide in the TGO is the alumina. A rough surface that does not form transient oxides or removal of transient oxides prior to TBC deposition will be a benefit in TBC adhesion. But, the rough surface formed by spraying of coarse particles tends to nucleate a higher amount of transient NiO and Cr2O3 in the scale.
Formation of alumina scale on the bond coat by pre-oxidation is known in the literature. In U.S. Pat. No. 6,123,997 preoxidized bond coats under defined temperatures and oxygen partial pressures to form alumina wherein the bond coat may also contain doped Pt or other noble metals. In yet another patent, U.S. Pat. No. 6,066,405, bond coats are used having an integrated bond coat with aluminum from 18 to 24 percent and integrated platinum content from 18 to 45 percent. U.S. Pat. No. 3,918,139 discloses a MCrAlY coating which comprises 3 to 12% of a noble metal selected from the group consisting of platinum or rhodium. The presence of platinum or rhodium greatly improves sulfidation resistance, and known to provide benefits to oxide adherence as well as reduce the propensity of forming transient oxides.
Furthermore, German Patent Document DE-A1-19842417 discloses a MCrAlY coating onto which a layer of pure platinum of 1 to 20 micrometer is deposited before it is coated with a ceramic coating. The platinum is applied for reasons of increased adherence of the Thermal Barrier Coating and the formation of a thin layer of aluminum oxide.
In addition, U.S. Pat. No. 5,942,337 discloses a multi-layered Thermal Barrier Coating for a superalloy article comprises a platinum enriched superalloy, a MCrAlY bond coating on the platinum enriched superalloy layer, a platinum enriched MCrAlY layer on the MCrAlY bond coating, a platinum aluminide coating on the platinum enriched MCrAlY layer, an oxide layer on the platinum aluminide coating and a ceramic Thermal Barrier Coating on the oxide layer.